Limiter circuits or limiters are extensively employed in the electronic field. A limiter is used if the level of an incoming signal is too high so that it can only be processed incompletely by subsequent processing circuits. An example of limiting action occurs when amplifiers are driven into cutoff and saturation whereby the gain of the amplifier is so great that the input signal causes limiting in both the positive and negative directions.
Limiters are often referred to as clipping circuits. Clipping circuits or limiters are used when it is desirable to select for transmission that part of an arbitrary waveform which lies above or below some particular reference voltage level. Limiters or clipping circuits are also referred to as voltage selectors or amplitude selectors. There are many different types which are employed, most of which utilize diodes.
A particular problem occurs when a limiter circuit is used in a television receiver. As is well known, television receivers have circuitry which is employed to control the beam current circulating in the television picture tube. The beam current delivered by the individual beam forming systems oftentimes are greater than the current which the television picture tube is designed to handle. In the case of a color television set, overloading results in blooming and color fringing in the bright areas. Thus, modern television receivers and monitors usually include a beam current limiting arrangement which controls the contrast characteristics of the video signals coupled to the picture tube in order to limit the beam current drawn by the picture tube. Beam current limiting is also referred to in the prior art as automatic beam limiting. The purpose of such a beam current limiting arrangement is to insure that excessive beam currents which may cause so-called "white spot blooming" and which may also damage the picture tube, are not provided. Typically, a beam current limiting arrangement senses the average current drawn by the picture tube from the high voltage transformer. The high voltage transformer supplies a high operating voltage to the picture tube, and the beam current limiting circuit generates a control signal for reducing the gain of the video signal processing channel to thereby reduce the peak-to-peak amplitude of the video signals coupled to the picture tube when the level of the beam current exceeds a predetermined threshold. The peak-to-peak amplitude of the video signals is related to the contrast of the image. Thus, such techniques for limiting the beam current are well known.
As indicated above, the effect caused by an excessive beam current mode is sometimes referred to as "blooming". Basically, "blooming" on a black and white scene is evidenced by changes in spot size and can result in changes in picture size, brightness and an overall distortion of the displayed scene. While blooming can occur in a black and white receiver, it can also occur in a compatible color receiver which, with automatic color control (ACC), will serve to degenerate the picture even further. See, for example, U.S. Pat. No. 3,578,903 issued on May 18, 1971 entitled "Control Circuits For Preventing Kiniscope Color Saturation During Blooming" by D. H. Willis. This patent explains the results of both blooming in monochrome and color television receivers.
Apart from the typical prior art types of beam current control circuits, limiter circuits are also necessary and employed in television receivers to control the beam current in the picture tube. The beam current in the picture tube is a function of the magnitude of the luminance signal which is provided, for example, in a color television system by the luminance amplifier. These amplifiers may become overloaded and may deliver currents to the picture tube which are larger than the currents which the color picture tube is designed to handle. Thus, the color picture tube, upon receiving such large signals, will overload resulting in "blooming" and color fringing which disrupts the picture, as described above. Tube limit values, which relate to the average beam current and the peak beam current, must not be exceeded for proper picture tube operation. For small image details, a considerably larger beam current than the average beam current can be tolerated by the color television tube for short times. Conventional limiter circuits in television receivers evaluate the average value and the short time value of the beam current by using threshold sensing circuits and therefore regulate the beam current to a lower value by means of employing a long and a short time constant. In this manner the time constants associated with the limiter circuits determines the attack and release time to enable a viewer to view a proper video signal. Apart from that use of limiting circuits, limiting circuits have been widely employed in television receivers to perform amplitude separation, such as being utilized in the sync separation portions of television receivers. Limiters have been used for noise reduction purposes in color television receivers. See U.K. Patent Application 2157909A published on Oct. 30, 1985 entitled "Noise Reduction Circuit for Composite Video Signals" by M. Kanda. This patent describes the use of a high pass filter, a notch filter, an amplitude limiter and an inverter to provide frequency selective noise reduction for a color television signal.
Limiter circuits which are employed in the luminance channel provide disadvantages in that the limiting adversely affects the entire signal reproduction capability of the receiver. The video signal content is not considered when limiting occurs and it is only the amplitude of the video signal that enables the limiting circuit to operate. As one can understand, a video signal contains many frequency components which relate to the picture content and which enables the picture to be provided in sharp or fine detail, depending on these video components. In various video frequency ranges the hard limiting will cause signal distortions which will adversely effect the reproduction of the signal. Such effects are even more noticeable if the video signals are digitized television signals. In such receivers the analog video signal is converted by means of an analog-to-digital converter to produce digital signals which are indicative of the video signal. In this manner, by utilizing limiting in the luminance channel one can cause aliasing effects which serve to transform into spurious signals which are undesired and which appear, and will be demodulated or otherwise presented, in the visible frequency range. Thus, in this manner, the use of hard limiting will produce spurious signals due to the frequency content of the limiter video signal which will produce interfering effects with the actual picture signal.
U.S. Pat. No. 3,529,244 entitled "Method and Apparatus for Frequency Sensitive Amplitude Limiting" issued on Sep. 15, 1980 to E. Torick et al. This patent describes an audio limiting circuit which separates a signal into low and high frequency components to selectively control gain and effecting preemphasis.
U.S. Pat. No. 4,064,537 entitled "Amplitude Limiting Circuit For Frequency Modulated Video Signals" issued on Dec. 20, 1977 to Y. Ota et al. This patent describes a limiter which uses a high pass and low pass filter. A first limiter amplifies and limits the high pass signals. The low pass signals are added to the limited high pass signals where the resultant output signal is then limited by a second limiter. The system is used in a magnetic tape, frequency modulated, video signal recording apparatus.
It is therefore an object of the present invention to provide a limiter circuit arrangement which produces spurious signals in frequency ranges which do not effect the desired signals.
It is a further object to provide a method for limiting signals wherein spurious signals that result from circuit operation and which are derived according to this method do not adversely effect the actual picture content as viewed.